feat: add frontend env var validation at build time

[darcszn]

Summary

Adds two-layer validation of required VITE_* environment variables so a
production build with a missing VITE_API_URL fails loudly instead of
shipping a silently broken app.

Changes

frontend/src/env.js (new)

• ENV_SCHEMA defines every VITE_* variable with a required flag, description,
  and optional format validator
• VITE_API_URL is required and validated as a well-formed URL
• VITE_VAPID_PUBLIC_KEY and VITE_CDN_URL are optional but validated if present
• Production: throws before the app renders if any required var is missing
• Development: console.warn only, so local work is unblocked without a full .env
• Exports API_URL, VAPID_PUBLIC_KEY, CDN_URL as named constants

frontend/src/main.jsx

• import './env.js' added as the very first import — runs before React,
  the store, or any component is touched

frontend/vite.config.js

• envValidationPlugin uses Vite's configResolved hook to fail vite build
  immediately if required vars are absent
• Switched base from process.env to loadEnv for consistent .env resolution

frontend/.env.example (new)

• Documents all VITE_* variables with required/optional labels and examples
• Points to both validation locations (src/env.js and vite.config.js)

Testing

• Missing VITE_API_URL in production build → vite build exits with clear error
• Missing VITE_API_URL in dev → console.warn, app still loads
• Invalid URL value → caught by the validate function in ENV_SCHEMA
• All optional vars absent → no errors, app renders normally
  closes Add frontend environment variable validation at build time #469

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